1. Field of the Invention
The invention relates generally to image capture for use in object sensor systems where the object is illumination sensitive. More specifically, one embodiment of the invention relates to strobed video capture for providing video images for use in biometric sensor systems.
2. Description of the Related Art
Imaging systems used in object monitoring or inspection are designed to generate accurate images of objects using a camera and sufficient illumination. In some instances, sufficient illumination to produce a useful image may be enough illumination to cause damage to, or otherwise alter, the object being imaged. One such object that can be damaged by excessive illumination is the human eye.
Biometric sensing techniques, where the human eye is monitored, have become useful in person identification and behavioral analysis. For example, iris recognition is a well known technique for identification, while pupillometry, the measure of a pupil's reaction to external stimuli, can be used for behavior analysis, e.g., detection of sleep deprivation, drug or alcohol abuse, and the like. To facilitate accurate biometric sensing using either iris recognition or pupillometry, images of the human eye must be accurately and clearly captured for analysis.
Commercial off-the-shelf video cameras typically run with a frame period of 33 ms, corresponding to a frame rate of 30 Hz. Often, eye movement occurs on that time scale. To avoid motion blur in images of the eye, the exposure time of each frame may be reduced using the electronic shutter capability found in many cameras. However, a reduction of ten times in shutter speed also reduces the signal-to-noise ratio (S/N) by ten times, all other parameters being held constant. If there are no restrictions on the irradiance of an object, the reduction in S/N and can be compensated for by increasing the scene irradiance (W/m2). However, when imaging an eye there are strict restrictions on irradiance for safety considerations.
Therefore, there is a need in the art for a method and apparatus for irradiating a scene to enable an imaging system to capture accurate images of an object, such as an eye, without damaging the object.